(#80) - Use nomnom for argument parsing

Why the switch
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1) optimist [has been deprecated](https://github.com/substack/node-optimist)
2) As part of #72 I'm likely to add new options soon (ones for cors e.g.) Having usage info in 2 places instead of three is handy then.
3) As part of #72 this project will become more complicated. Things like binding to a new port/ip while running need to be supported, as well as changing the PouchDB object while running. Less global variables and validation of these variables later on will definitely help then. Nomnom helps in both cases.
I mean: needs to be fixed before this PR is merged.